The major intent behind controlled rolling would be to refine grain structure and, thereby, to improve the strength and toughness of steel inside the as-hot-rol1ed condition. If a survey is made from the growth of controlled rolling, it could be seen that controlled rolling consists of three stages: (a) deformation from the recrystallization region at high temperatures; (b) deformation in the non-recrystallization region in a low temperature range above Ar3; and (c) deformation in the austenite-ferrite region.
It is actually stressed that the importance of deformation from the nonrecrystallization region is dividing an austenite grain into several blocks by the introduction of deformation bands inside it. Deformation in the austenite-ferrite region provides a mixed structure consisting of equiaxed grains and subgrains after transformation and, thereby, it improves further the strength and toughness.
The basic distinction between conventionally hot-rolled and controlled -rolled steels depends on the reality that the nucleation of ferrite occurs exclusively at austenite grain 34dexppky from the former, although it takes place in the grain interior and also at grain boundaries from the latter, leading to a much more refined grain structure. In Hot Rolled Steel Plate a crystallographic texture develops, which in turn causes planar anisotropies in mechanical properties and embrittlement within the through -thickness direction.
The latter is demonstrated to be the main cause of the delamination which appeared from the fractured Charpy specimens. Fundamental aspects of controlled rolling, for example the recrystallization behaviour of austenite, the retardation mechanism of austenite recrystallization due to niobium, microstructural changes accompanying deformation, factors governing strength and toughness, etc., are reviewed. The technique of controlled rolling in plate and strip mills is outlined.